1. Field of the Invention
This invention relates to an air-fuel ratio (e.g., oxygen concentration) sensor provided in an exhaust gas system for an internal combustion engine of an automotive vehicle.
2. Description of Related Art
Gas concentration detectors are incorporated in many of recent internal combustion engines for automotive vehicles to detect and feedback control the air-fuel ratio of the gas mixture introduced into their combustion chambers. The air-fuel ratio by a gas concentration detector is generally used to control the combustion of air/fuel mixture in the combustion chamber to emit exhaust gas suitable for a attaining an optimum gas purification efficiency in the exhaust gas system comprising a catalytic converter.
An ordinary oxygen concentration detector comprises an oxygen concentration sensing element with a ZrO.sub.2 solid electrolyte and a housing accommodating this oxygen concentration sensing element. Oxygen concentration sensing elements are roughly classified into listing current type detectors and oxygen concentration cell type detectors.
FIGS. 34 and 35 cooperatively show one example of the oxygen concentration sensing element which has a solid electrolyte of a cup shape, i.e., a cylindrical shape having a bottom.
More specifically, an oxygen concentration sensing element 9 comprises a cup-shaped solid electrolyte 90, an external electrode 95 provided on an outer wall surface of solid electrolyte 90, an internal electrode 96 provided on an inner wall surface of solid electrolyte 90, and an insulating layer 91 provided on the surface of external electrode 95.
Furthermore, oxygen concentration sensing element 9 has an inside hollow space defining an inside chamber 92 for introducing a reference gas. A round stick-like heater 99 is inserted into this inside chamber 92 and held therein. Insulating layer 91, capable of serving as a protecting layer for the external electrode 95, is formed by a ceramic coating layer. Insulating layer 91 may be a complex layer comprising, for example, a .gamma.-Al.sub.2 O.sub.3 layer formed on the ceramic coating layer.
To clear the regulations for exhaust gas emission which yearly become more severe, it is required to control the air-fuel ratio more precisely to realize an optimum combustion in an automotive vehicle's internal combustion engine. In this respect, developing an accurate oxygen concentration sensing element is a key factor to realize such an excellent air-fuel ratio control.
For example, among such advanced oxygen concentration sensing elements, there is an oxygen concentration sensing element capable of quickly warming up to detect an oxygen concentration within a short period of time from a start-up operation of the automotive vehicle's internal combustion engine.
In general, every oxygen concentration sensing element has an element active temperature. At or above this element active temperature, the oxygen concentration sensing element can operate normally. Immediately after the internal combustion engine is started, temperatures of the exhaust system and its peripheral components are relatively low. Accordingly, the heater inserted in the inside chamber is actuated to increase the temperature of the oxygen concentration sensing element promptly to its element active temperature. This is why the heater is necessary to warm up the sensing element.
In view of the thermal efficiency during a warm-up operation of an oxygen concentration sensing element, it is generally advantageous to provide a heater integrally with the sensing element, rather than providing a separate heater. Such an oxygen concentration sensing element is, for example, disclosed in Unexamined Japanese Patent Application No. SHO 58-76757, published in 1983, wherein an electrically activated heater layer is provided on a wall surface of a solid electrolyte (except a region of an external electrode).
However, according to the oxygen concentration sensing element disclosed in this prior art, there is a problem that the temperature of the solid electrolyte is excessively increased when a higher voltage is applied to the heater layer, because the heater layer is provided directly on the surface of the solid electrolyte. In this case, components constituting the solid electrolyte, such as ZrO.sub.2, may be subjected to reduction and decomposition due to the applied electrical potential and heat. This leads to a deterioration in the insulation property between the external electrode and the internal electrode, causing a problem of inaccurate detection of the oxygen concentration. Moreover as, the solid electrolyte is deteriorating, a leak current flows from the heater layer into an oxygen concentration detecting circuit.
The oxygen concentration detection circuit, in this case, is an electric circuit for judging a concentration of oxygen involved in the measured gas based on an output signal (i.e., electric voltage or electric current) generated from the oxygen concentration sensing element. The external electrode and the internal electrode, as well as later-described external electrode lead, internal electrode lead, external electrode terminal and internal electrode terminal, respectively constitute part of the oxygen concentration detection circuit.
Furthermore, according to the oxygen concentration sensing element disclosed in the prior art, a distance between the heater and the external or internal electrode is not short when compared with that of a conventional separate type heater. Hence, its warm-up ability is not satisfactory.
Unexamined Japanese Utility Model Application No. SHO 59-95257, published in 1984, discloses another oxygen concentration sensing element with an integral heater, wherein a coil-like heater is provided on the outer cylindrical surface of the oxygen concentration sensing element.
However, according to this prior art, a relatively large amount of thermal energy is transmitted or radiated to the outside because the heater is provided on the outer cylindrical surface. The percentage of thermal energy actually transferred to the oxygen concentration sensing element is small. Hence, its warm-up ability is not satisfactory.